It is particularly important to test the signals generated by a scanner of a position measuring device in order to correctly install the scanner in the position measuring device. It is well known to provide a position measuring device having a scanner and scale to determine the position of a movable part, such as a machine, relative to a fixed part. Typically, the scanner outputs two signals in quadrature. These outputs can be input to further circuitry for counting the cycles of the incoming signals so as to determine position, to determine the direction of movement, and to provide position interpolation within one cycle of the output. It is desirable that the scanner be correctly installed relative to the scale so that the output signals from the scanner are accurately in quadrature with a 90.degree. phase shift and have substantially the same amplitude. Signal quality and the precision of measurement depends greatly on the correct installation of the scanner relative to the scale. The amplitude of the signals generated by the scanner is influenced by the distance between the scanner and scale and the phase shift is influenced by the tilt of the scanner relative to the direction of measurement.
With incremental measuring systems, in addition to the incremental division, a reference mark or marks is placed on the scale. When the reference mark is scanned, an analog reference mark signal is generated which defines the reference mark's position. As a rule, with the appearance of the maximum of the reference mark signal, a stored absolute reference position is assumed by a counter and this assumed value is continuously updated by the incremental position measuring occurring during the relative movement between the scale and the scanner. Because an incremental counting step is only a fraction of a period of the scale, it is particularly important that the reference mark signal is associated as favorably as possible with a particular counting step, i.e., fraction of the period. This association is influenced considerably by the angular orientation of the scanner.
In order to test the correct installation of the scanner relative to the scale it is known to display the amplitude and phase shift of the analog signals generated by a scanner on an oscilloscope. The two analog signals output by the scanner are input to two channels of a double beam oscilloscope so that a Lissajous figure is generated. The radius of the Lissajous figure is a measure of the amplitudes as well as the phase relation of the analog signals. If the scanner is perfectly positioned with respect to the scale, the Lissajous figure will be a perfect circle centered at the origin.
In order to simplify the testing of the signals generated by a scanner, U.S. Pat. No. 5,128,609 discloses a control device having a bar display. In this control device, a radius of the Lissajous figure is calculated from the instantaneous amplitudes of the analog signals generated by a scanner and the instantaneous radius value is represented as a lighted point on the bar display. While this control device is easier to handle than an oscilloscope, the bar display does not provide sufficient resolution to allow one to make fine adjustments in the scanner's position.
During the installation of a scanner relative to a scale, the amplitudes of the analog signals generated by the scanner can range from zero to a maximum of, for example, 12 .mu.A. This relatively large range must be able to be displayed in order for the analog signals to be tested. However, because of space limitations displaying such a large range compromises the resolution of the display. This has the disadvantage that the range of variation of the radius of the Lissajous figure cannot be represented with sufficient resolution and precision since the instantaneous radius value appears only as a single lighted point. In addition, there is no disclosure of testing a reference mark signal.
The RENISHAW company sells a control device for testing position-dependent scanning signals. This control device with the name RGSUE has features according to U.S. Pat. No. 5,128,609 described above. Furthermore, this control device has a display field for indicating the position of a reference mark. In the operating instructions, it is explained that the detection of the reference mark can be checked using the display. An adjusting screw is provided for adjusting the position of a reference mark. When the position of the reference mark is correctly adjusted, a green LED is illuminated on the display; when the position of the reference mark is incorrectly adjusted, a yellow or red LED is illuminated. This control device has the disadvantage that its ability to assist the operator in adjusting the scanner particularly during installation is insufficient. This disadvantage also applies to the device described in U.S. Pat. No. 5,241,173.
An object of the present invention is to provide a control device and a process for testing position-dependent analog signals where the parameters of the analog signals can be displayed with sufficiently high resolution and precision in order to make an adjustment of the scanning head easier for the operator.
An advantage of the present invention is that important parameters of the analog signals can be better recognized and thus, in a simple way, a good quality of analog signals can be obtained. Furthermore, the adjustment of the scanner is made easier during installation by virtue of the fact that the direction in which the scanning head should be moved and pivoted is also displayed.